Compressor recirculation into annular volume

ABSTRACT

To solve the problems of compressor wheel blade flow separation causing surge type noises when an exhaust gas recirculation (EGR) duct introduces into the compressor exhaust gases to be returned to the engine, the exhaust gas is fed into an annular volume, defined between inner and outer walls or an annular transition cavity shaped as a radially expanded, axially flattened cylindrical space in the compressor inlet, so that the generally unidirectional radial flow from an EGR duct is re-directed and organized as it is turned from generally radial to generally axial, merging with the general inlet flow and presenting the compressor wheel with airflow of “circumferentially uniform” flow velocity.

FIELD OF THE INVENTION

This invention is directed to the configuration of a turbocharger duct,integrated into a compressor cover, delivering recirculation air from acompressor discharge to a compressor inlet, in such a manner as to notcause surge events or noise.

BACKGROUND OF THE INVENTION

Turbochargers deliver air, at greater density than would be possible inthe normally aspirated configuration, to the engine intake, allowingmore fuel to be combusted, thus boosting the engine's horsepower withoutsignificantly increasing engine weight. A smaller turbocharged enginecan replace a normally aspirated engine of a larger physical size, thusreducing the mass and aerodynamic frontal area of the vehicle.

Turbochargers are a type of forced induction system which uses theexhaust flow entering the turbine housing from the engine exhaustmanifold to drive a turbine wheel (51) which is located in a turbinehousing (2). The turbine wheel is solidly affixed to a shaft to becomethe shaft and wheel assembly. The primary function of the turbine wheelis extracting rotational power from the exhaust gas and using this powerto drive the compressor.

The compressor stage consists of a wheel (20) and it's housing (10). Thecompressor wheel (20) is mounted to a stub shaft end of the shaft andwheel assembly and is held in position by the clamp load from acompressor nut. Filtered air is drawn axially into the inlet (14) of thecompressor cover by the rotation of the compressor wheel at very highRPM. The turbine stage drives the compressor wheel to produce acombination of static pressure with some residual kinetic energy andheat. The pressurized gas exits the compressor cover through acompressor discharge (15) and is delivered, usually via an intercooler,to the engine intake. Compressor surge occurs when the compressorattempts to deliver more massflow to the engine than is possible at theexisting engine operating condition, i.e., aerodynamic stall. Thecompressor stage begins to oscillate in terms of pressure: mass flow,speed, and net aerodynamic thrust. This oscillating instability can bequite damaging to the turbocharger, also producing an irritating noisewhich can be heard by the driver and is usually described as a “bark” or“squawk”. The location of surge for a given compressor design can bedescribed as a function of pressure and mass flow at a given rotationalspeed.

Compression ignition (CI) engines have air induced directly into thecylinder. The air is compressed by the piston on the compression stroke,and the fuel is injected into the heated compressed air just before thepiston reaches top dead center (TDC).

In turbocharged CI engines the mass flow of air is delivered by theturbocharger output, and the fuel flow is metered and injected directlyinto the combustion chamber. Some CI engines are equipped withthrottles.

Spark Ignited (SI) engines may mix the combustion air with fuel in theinlet manifold. The resultant air-fuel mixture is controlled by athrottle valve prior to entering the combustion chamber. The throttlevalve, or plate, is typically located in a throttle body with relativelyclose tolerances and has the ability to close off the airflow to theengine. SI engines may also inject fuel directly into the cylinder.

“Tip-in” is the term used to refer to the action of driver's footdepressing the accelerator pedal to adjust engine load. Engine speed mayremain the same, driving up a hill, for example, or the engine mayincrease from low engine speed to higher engine speed. “Tip-out” is theterm used to refer to the opposite action of the driver's foot liftingoff the pedal.

Compressor Recirculation Valves (CRV) and the ducts connecting theexhaust from the compressor to the inlet to the valve and the exhaustfrom the valve to the inlet of the compressor, collectively hereafterthe “CRV system”, are used today in many SI and CI engines, or enginesemploying throttle plates for air control, typically to prevent surge.The closing of the throttle plate, at accelerator pedal tip-out, forexample, closes the duct from the compressor discharge to the engineinlet and causing a sudden reduction in compressor flow resulting in thecompressor stage to going into surge. CRV systems in general deliver airfrom the compressor discharge duct (the duct connecting the compressordischarge to the engine inlet or intercooler, depending upon the engineconfiguration) to the ducting upstream of the compressor inlet, ordirectly to the compressor inlet.

The output of the compressor recirculation valve may be connected to thecompressor inlet by ducting to the compressor inlet or ducting to thecompressor wheel. Alternatively, the compressor recirculation valve andducting may be part of the compressor cover casting. Similarly the inputto the compressor recirculation valve may be a duct connecting the CRVto the compressor discharge, or the input to the CRV may be part of thecompressor cover.

The CRV system may be connected to the compressor by ducting to thecompressor in several ways. Some systems have piping from the compressordischarge to the CRV valve, and then from the CRV valve to the ambientinlet duct from the air cleaner or even the compressor inlet duct; somehave parts of this arrangement only (for example the CRV may be mountedto the compressor cover discharge with a flexible pipe to the compressorinlet); some have the CRV integrated directly into the compressor cover.The invention teaches the method of directing the recirculated air tothe compressor wheel, regardless of the design by which the CRV assemblymay be mounted.

When the CRV valve is opened, a volume of high pressure compressordischarge air radially enters the compressor inlet, joining the axiallyflowing main stream, causing a sudden inrush of air through the aperturein the compressor cover inducer (defined below) and can cause cavityresonance noise akin to that of blowing over the opening in the neck ofa bottle. This noise may also be irritating to the driver. In FIG. 2Athe inlet air (61) is sucked axially into and through the compressorwheel (20) compressed and ejected from the compressor wheel through thediffuser (11) into the volute (12), where the velocity component of airfrom the compressor wheel and the diffuser is collected and translatedto pressure. Typically there exists a CRV (80) which controls arecirculation flow of pressurized air (62) from the compressor discharge(15). With the CRV (80) in the open position, or modulated towards theopen position, the recirculation air (64) is admitted to a recirculationduct (16) and thence admitted into the compressor inlet (e.g., theregion directly upstream of the compressor wheel leading edge (24).

Because the duct (16), when fluidly connecting the compressor discharge(15) to the CRV; and the CRV to the compressor stage inlet (14); dumpsthe unidirectionally, generally radially flowing, high pressurerecirculation air directly into the axially flowing main inlet airflow(61) and then to the inducer area (14 a) the velocity distributionacross the plane of leading edges (24) of the compressor wheel is notuniform. This lack of circumferentially uniform airflow velocity issucked into the compressor wheel which results in some flow aligned withthe blades and some flow not aligned with the blades. Some blades arefully loaded while some blades may be in a negative pressure condition.

FIG. 2B is a view of the section “A-A” of FIG. 2A. In FIG. 2B, thegenerally radial airflow is depicted in plan view. In this section view,the airflow (64) from the discharge duct (15, 15 a) is introduced intothe CRV duct (16) connecting the CRV to the compressor inlet. Thegenerally radial airflow (65) from the CRV duct dumps directly into theaxial main compressor inlet flow (61) without the opportunity for themerged airflow to be conditioned; thus, the air ingested by thecompressor wheel is generally unconditioned from a velocity standpoint.

This duct configuration causes a pressure gradient across the inlet tothe compressor wheel which, when severe, can cause an uneven bladeloading of the compressor wheel blades. This excitation can result inhigh cycle fatigue (HCF) of said blades. In addition this ductconfiguration also causes the surge noise described above. Thisphenomenon also causes a reduction in stage efficiency when the CRV isopen.

So it can be seen that “dumping” compressor recirculation flow directlyinto the compressor wheel causes not only noise, but also the potentialfor compressor wheel blade failure and a general loss of efficiency whenthe CRV s open.

SUMMARY OF THE INVENTION

There existed a need to solve the problems of noise generated during theCRV opening and or closing event(s).

The inventors solved the problems by providing an annular transitioncavity surrounding the inducer, in which to condition the mergingairflow and generate airflow of uniform velocity and direction to theinducer and thus the compressor wheel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not bylimitation in the accompanying drawings in which like reference numbersindicate similar parts and in which:

FIG. 1 depicts a section of a typical turbocharger assembly;

FIG. 2A depicts a section of a compressor cover with a recirculationvalve;

FIG. 2B depicts a section A-A of the cover of FIG. 2A;

FIG. 3A depicts a section of the inventive compressor cover;

FIG. 3B depicts a section B-B of the cover of FIG. 3A;

FIG. 4A depicts a section of the second embodiment of the invention”;

FIG. 4B depicts a section B-B of the cover of FIG. 4A;

FIG. 5A depicts a variation of the first embodiment of the invention;

FIG. 5B depicts a section C-C of the cover of FIG. 5A;

FIG. 6A depicts a variation of the second embodiment of the invention;and

FIG. 6B depicts a section B-B of the cover of FIG. 6A.

DETAILED DESCRIPTION OF THE INVENTION

To the aerodynamicist, airflow through a compressor stage can be quitecomplex to describe with a high degree of precision. However, forpurposes of understanding the principles of the present invention, thediscussion of airflow can be greatly simplified. It is simply necessaryto understand that in the inventive duct the air fed from the CRV ductinto the compressor inlet does not enter from one direction (i.e., asdoes the duct outlet in the conventional CRV system), but is caused tobe distributed generally uniformly about the circumference of thecompressor inlet prior to being introduced into, and merged with, themain axial airflow. In the following, the inventive IGV system design,and the airflow through the inventive compressor, will be described ingreater detail, but when discussing flow will use only the maindirections of flow. In general, artifacts such as swirl, whirl, andturbulence will not be taken into consideration. Unless otherwiseindicated, the following terms shall have the following meanings:

“Generally radial” refers to the component of flow generally flowingaway from, or converging towards, the axis which is a continuation ofthe compressor wheel axis, and includes flow having some non purelyradial component (e.g., axial component or tangential component.

“Generally axial” refers to the component of flow generally flowingparallel to the axis of the turbocharger, even if the flow has somewhirl or non parallel component

“Upstream” and “downstream” are with reference to the major component offlow, i.e., flow going from compressor inlet to compressor wheel. Theseterms do not refer to immediate flow in the CRV duct, the annularcavity, or the radially compressed, axially expanded volume extendingfrom the compressor inlet.

“Axially flattened, radially expanded volume” means that the shape ofthe typically cylindrical, but frequently curved or non-cylindrical,channel wall (22) is modified by being expanded radially, with radialexpansion greater than axial height.

The inventors determined through testing that for 0.03 seconds duringthe opening of the CRV there was a squawking sound. ThroughComputational Fluid Dynamics (CFD) modelling it was found that thenoises were due to the dumping of unconditioned recirculation air fromthe compressor discharge into the lower compressor inlet and thence toinducer of the compressor wheel. The primary velocity component of theflow from the CRV duct is unidirectional or nearly unidirectional, andgenerally radial, thus generally perpendicular to the axial velocity ofthe main compressor flow. In order for the compressor wheel blades to beuniformly loaded, the (generally unidirectionally radially) CRV flow hasto be converted to generally uniform axial velocity before it isingested into the compressor wheel. Without something to channel theflow and get it uniformly, i.e., evenly around 360°, flow transitionedand thus aligned with the axial main compressor inlet flow, the CRV flowdoes not mix well; so the compressor wheels ends up ingesting highlydisorganized flow. Some blades are fully loaded while some blades arepractically in a vacuum.

As depicted in FIGS. 2A, 2B, air compressed by the compressor wheel (20)is driven into the diffuser (11) and into the volute (12) where velocityof air from the compressor and diffuser is collected. This air is thendirected to the compressor discharge (15) where it typically connects toa duct directing the air to the vehicle intercooler and thence theengine intake manifold. When the CRV (80) is opened, some portion of theair (63) in the compressor discharge passes through a CRV duct (16),fluidly connecting the high pressure compressor discharge (15) with the(relatively) low pressure compressor inlet (14). The airflow (64, 64 a)into the duct (16) is controlled by the position of the CRV (80). Withthe CRV open, the airflow (64) in the CRV duct “dumps” unidirectionallyand generally perpendicular to the main axis of flow into the compressorinlet (14). It was found that the airflow in the compressor inlet (14)was not uniformly distributed as it impinged on the plane of the leadingedges (24) of the compressor wheel. The inventors determined that boththe direction of the incoming flow and the circumferential distributionabout the compressor wheel axis were not uniform, resulting in undesiredblade excitation with the potential for high cycle fatigue (HCF) of thecompressor wheel blades, coupled with an efficiency loss for the periodof each CRV event. It was also reported that for the period of CRVopening and closing, there were undesirable noises coming from theturbocharger. These noises were traced to the CRV events. The inventorsdesigned a new configuration for the inlet, from the CRV recirculationto the compressor wheel inducer, such that the incoming “dump” of highpressure air produced neither the “whoosh” of the air through theopening in the compressor inlet, nor the “squawk” due the surge event asthe CRV closed at the conclusion of the event.

In the first embodiment of the invention, as depicted in FIGS. 3A, 3B,the compressor housing includes a compressor inlet (14) upstream of thecompressor wheel and a compressor discharge (15) downstream of saidcompressor wheel, the inlet comprising an outer channel wall (22)extending away from the compressor wheel in an upstream direction andforming a gas intake portion in the upper inlet section; an innerchannel wall (21) within the outer channel wall, comprising a lowerinlet section extending away from the compressor wheel upstream of theleading edges (24) of the compressor wheel and an inducer section (14 a)downstream of the leading edges (24) of the compressor wheel; and anannular cavity (23) defined between the inner and outer channel walls,with an annular opening of the annular cavity (23) at the upstream endof the inner channel wall between the inner and outer channel walls, thecompressor further comprising a compressor return valve system includinga compressor recirculation valve (CRV)(80) and a recirculation ductfluidly connecting the compressor discharge (15) to the annular gaspassage.

The generally cylindrical inside surface of the inner wall (21) is knownas the inducer (14 a) (for the zone where the wall (14 a) is adjacent tothe compressor wheel profile (27)), and the inlet (14)) is above theleading edges of the compressor wheel blades). The outside surface ofthis generally cylindrical inner wall is the inside wall of the annularcavity (23). The outside of the generally cylindrical annular cavity(23) is the generally cylindrical inside surface of the outer wall (22).

The lower bound of the generally annular cavity is typically defined bycasting coring requirements and the desire to have some volume below theentrance of the CRV duct (16) into the annular cavity (23). The upperbound of the annular cavity is unbound but for the height of the innerwall (21), which should be maintained as close as possible to that ofnot less than 0.5 D_(i), where D_(i) is the inducer diameter. The heightof this wall has an effect on map width.

FIGS. 3A and 3B depict an outflow (62) of compressed air from thecompressor (20) to the intercooler and engine. Recirculation airflow(63) flows into the CRV duct (16). A CRV (80) controls the passage ofrecirculation air (64) through a recirculation duct (16), fluidlyconnecting the compressor discharge (15) with the compressor inlet (14).With the CRV (80) in an opened condition, compressor dischargerecirculation airflow (63, 64) travels in the CRV duct (16).Predominantly radial airflow (64), from the CRV duct (16) flows into,and around (66), an annular cavity (23). Airflow (64) from the annularcavity, now evenly distributed around the 360° of the compressor inletwall (21) flows in a generally axial direction up, and over (68) theupper end of the inner wall (21), merging with the generally axialincoming flow of air (61) from an engine air filter and which presentsthe compressor wheel leading edges (24) with a flow of air (70 plus 61),which is now circumferentially uniformly distributed across the plane ofthe leading edges (24) of the compressor wheel.

In a variation to the first embodiment of the invention, as depicted inFIGS. 4A and 4B, in the case in which greater map width is required,this can be provided by resort to an inducer recirculation compressorcover. In this case the annular cavity (23) serves a dual function. Thefirst function is that of a volume and a pathway for compressor inducerrecirculation; the second function is as that of the first embodiment ofthe invention. In order to support the generally cylindrical part of theinner wall (21) which, in this embodiment is separated from the lowerpart (21 a) of the inner wall by a recirculation slot (25), struts (94)are provided. The struts (94) are typically part of the casting andserve no structural purpose until the inducer recirculation slot (25) iscut through the inner wall (21), thus separating the upper part of theinner wall from the lower part of the inner wall.

The struts are typically vertical, axial and linear in section but couldbe tilted, or non linear, to promote a more uniform flow over the upperend of the inner wall into the compressor wheel.

In another variation to the first embodiment of the invention, thestruts are tilted or twisted such that flow from the CRV duct has somerotation (about the compressor wheel centerline) to it from: eithersolely the orientation and design of the struts; a solely tangentialdischarge from the CRV duct into the annular cavity; or withcontribution from both design elements. Since pro-rotation (with respectto the rotation of the compressor wheel) of the air entering thecompressor wheel causes a map shift, moving the map away from surge, theopening of the CRV valve helps prevent surge for the transient period ofthe CRV opening event.

In a second embodiment of the invention, as depicted in FIGS. 5A and 5B,the airflows (63, 64, 64 a) from the compressor discharge (15),controlled by the CRV (80), do not feed into a separate annular cavitysurrounding the inner wall (21), but feed generally radially into anaxially flattened, radially expanded cylindrical volume (hereafter“expanded cylindrical volume”) located about the compressor inlet,axially located between the inlet to the compressor cover and theleading edge of the compressor wheel. The cylindrical volume is bound bya generally cylindrical outer wall (92) and an upstream (relative to themain axis of flow (61)) generally planar wall (90) generallyperpendicular to the main axis of flow and a downstream generally planarwall (91) also generally perpendicular to the main axis of flow. Thesewalls are described as generally perpendicular and generally parallel tothe axis of flow to allow for casting draft angles. In the preferredmode, the upstream and downstream, generally perpendicular, walls areflat but they could be defined by curves. The critical function of theflattened expanded cylindrical volume is that of airflow, although froma production standpoint, it is critical that the tooling is retractable.

As in the first embodiment of the invention, as described above, thelower bound of the generally annular cavity is typically defined bycasting coring requirements and the desire to have some volume below theentrance of the CRV duct (16) into the annular cavity (23). The upperbound of the annular cavity is unbound but for the height of the innerwall (21), which should be maintained as close as possible to that ofnot less than 0.5 D_(i), where D_(i) is the inducer diameter. The heightof this wall has an effect on map width.

The outflow (62) of compressed air from the compressor (20) goes to theintercooler and engine. Recirculation airflow (63) flows into the CRVduct. A CRV (80) controls the passage of recirculation air (64) througha recirculation duct (16), fluidly connecting the compressor discharge(15) with the expanded cylindrical volume. With the CRV (80) in anopened condition, compressor discharge recirculation airflow (63, 64 64a) travels in the CRV duct (16). Airflow from the CRV duct flowsgenerally radially into, and then around, the expanded cylindricalvolume. Airflow (67) from the expanded cylindrical volume is turned fromradial, or generally radial, to generally axial as it traverses theexpanded cylindrical volume and enters into the lower part of thecompressor inlet, where it merges with the axial main compressor inletflow (61), and then to the compressor wheel leading edges (24). As thegenerally radial airflow (67) from the CRV duct is turned into thegenerally axial airflow (70) headed for the compressor wheel, theairflow acquires some axial velocity from the main compressor inlet flow(61) to enable the compressor wheel to ingest highly organized flow,which produces relatively uniform blade loadings.

While in the preferred mode of the second embodiment of the invention,the outer generally vertical wall of the cylindrical volume is generallycylindrical, and the upper and lower walls of the cylindrical volume aregenerally flat and generally horizontal. In a variation to the secondembodiment of the invention, said walls may be more non-symmetricalcomplex shapes, defined by the entry conditions of the duct from the CRVto the cylindrical volume, such that the outflow from the cylindricalvolume towards the compressor wheel leading edge is circumferentiallyuniformly distributed to the lower part of the compressor inlet.

In FIGS. 6A and 6B, the inventors devised a more complex shape (98) tomanipulate the generally radial airflow (62) from the CRV duct into thegenerally axial, circumferentially uniformly distributed, airflow (67and 65) into the lower part of the compressor inlet where it merges withthe axial main compressor inlet flow (61) and then to the compressorwheel leading edge (24). This airflow change in direction and nature isexecuted with a non-symmetric geometry for both the outer generallyvertical wall (98) of the cylindrical volume and the upper (96) andlower (97) generally horizontal walls of the cylindrical volume. As thegenerally radial airflow (69) from the CRV duct is turned into thegenerally axial airflow (70) headed for the compressor wheel, theairflow acquires some axial velocity from the main compressor inlet flow(61) to enable the compressor wheel to ingest highly organized flow (61,70) which produces relatively uniform blade loadings.

The inventive concept of transitioning the generally unidirectionalradial recirculation airflow from the compressor recirculation valvethrough an annular volume so that it is distributed circumferentiallybefore it reaches the generally axial airflow of the compressor inlet,merging uniformly with the general axial inlet flow and presenting thecompressor wheel with airflow of “circumferentially uniform” flowvelocity, is not limited to the recirculation of air within thecompressor, but is applicable to solving similar situations whereuniformity of introduction of gas from a radial flow into an axial flowis desired, e.g., the introduction of exhaust gas recirculation (EGR)from an engine exhaust manifold into the air flow upstream of acompressor. Here also, by being transitioned prior to merging, theproblem of compressor wheel blade flow separation can be overcome.

In invention comprises a low pressure EGR system for an internalcombustion engine comprising: a compressor for compressing air to besupplied to the engine; a turbine stage including a turbine housinghaving an exhaust gas inlet for receiving exhaust gas from said engine,and an exhaust gas outlet, and a turbine wheel driven by the exhaust gasfrom the engine; an exhaust gas return duct by which exhaust gases to bereturned to the engine are introduced to the compressor; and a boost airduct by which the air compressed in the compressor is supplied to theengine, wherein the compressor comprises: a compressor wheel (20) drivenby the turbine wheel and including a plurality of blades each of whichincludes a leading edge (24), a trailing edge and an outer free edge,said wheel being mounted for rotation within a compressor housing andoperable between choked flow and a surge line, the compressor housingincluding a compressor inlet (14) upstream of the compressor wheel and acompressor discharge (15) downstream of said compressor wheel, whereinthe inlet comprises a channel wall (22) extending away from thecompressor wheel in an upstream direction, and including an axiallyflattened, radially expanded cylindrical volume annularly open to thecompressor inlet (16), axially upstream of the leading edge of thecompressor wheel, and wherein said exhaust gases to be returned to theengine are introduced via the exhaust gas return duct into the axiallyflattened, radially expanded cylindrical volume in the compressor inlet(16).

Now that the invention has been described, what is claimed is:
 1. A lowpressure exhaust gas return (EGR) system for an internal combustionengine comprising: a compressor for compressing air to be supplied tothe engine; a turbine stage including a turbine housing having anexhaust gas inlet for receiving exhaust gas from said engine, and anexhaust gas outlet, and a turbine wheel driven by the exhaust gas fromthe engine; an exhaust gas return duct by which exhaust gases to bereturned to the engine are introduced to the compressor; and a boost airduct by which the air compressed in the compressor is supplied to theengine, wherein said compressor comprises: a compressor housingincluding a compressor inlet (14) and a compressor discharge (15), saidcompressor inlet (14) being defined by a channel wall (22) having afirst circumference and having a center, a compressor wheel (20) mountedfor rotation within said compressor housing between said compressorinlet (14) and compressor discharge (15), an annular cavity definedbetween inner and outer channel walls, or an annular transition cavityforming by a radially expanded cross-sectional area of the channel wallin the compressor inlet, arranged about the entire circumference of thecompressor inlet (14) for introducing air from the compressorrecirculation duct (16) into the compressor inlet (14) about the entirecircumference of the compressor inlet (14), and wherein said exhaustgases to be returned to the engine are introduced via the EGR duct intothe annular cavity.
 2. A low pressure exhaust gas return (EGR) systemfor an internal combustion engine comprising: a compressor forcompressing air to be supplied to the engine; a turbine stage includinga turbine housing having an exhaust gas inlet for receiving exhaust gasfrom said engine, and an exhaust gas outlet, and a turbine wheel drivenby the exhaust gas from the engine; an exhaust gas return duct by whichexhaust gases to be returned to the engine are introduced to thecompressor; and a boost air duct by which the air compressed in thecompressor is supplied to the engine, wherein said compressor comprises:a compressor housing including a compressor inlet (14) and a compressordischarge (15), said compressor inlet (14) being defined by a channelwall (22) having a first circumference and having a center, a compressorwheel (20) mounted for rotation within said compressor housing betweensaid compressor inlet (14) and compressor discharge (15), a compressorrecirculation duct (16) providing fluid communication between thecompressor discharge (15) and the compressor inlet (14), a compressorrecirculation valve (80) for control of air flow in said compressorreturn recirculation duct, an annular transition cavity formed by aradially expanded cross-sectional area of the channel wall (22) arrangedabout the entire circumference of the compressor inlet (14) forintroducing air from the compressor recirculation duct (16) into thecompressor inlet (14) about the entire circumference of the compressorinlet (14), and wherein said exhaust gases to be returned to the engineare introduced via the EGR duct into the annular transition cavity. 3.The system as in claim 2, wherein said radially expanded cross-sectionalarea of the channel wall (22) is co-axial with said compressor inlet(14) channel wall (22) having said first circumference.
 4. The system asin claim 2, wherein said annular transition cavity formed by a radiallyexpanded cross-sectional area of the channel wall (22) is formed by aradially expanded channel wall (22) having a center that is not co-axialwith said compressor inlet (14).
 5. The system as in claim 2, furthercomprising at least one flow directional control device located in saidannular transition cavity.
 6. The system claim 2, wherein gas from theEGR tangentially enters the annular transition cavity.
 7. A low pressureexhaust gas return (EGR) system for an internal combustion enginecomprising: a compressor for compressing air to be supplied to theengine; a turbine including a turbine housing having an exhaust gasinlet for receiving exhaust gas from said engine, and an exhaust gasoutlet, and a turbine wheel driven by the exhaust gas from the engine;an EGR duct by which exhaust gases to be returned to the engine areintroduced to the compressor; and a boost air duct by which the aircompressed in the compressor is supplied to the engine, wherein saidcompressor comprises: a compressor wheel (20) including a plurality ofblades each of which includes a leading edge (24), a trailing edge andan outer free edge, said wheel being mounted for rotation within ahousing, the housing including a compressor inlet (14) disposed on oneside of the compressor wheel so as to direct gas flow to the leadingedges of the blades, the inlet defining an inlet axis that is parallelto a rotational axis of the compressor wheel, a diffuser on another sideof said compressor wheel so as to receive gas flow from the compressorwheel, and a volute that receives gas flow from the diffuser anddelivers it to a compressor discharge (15), wherein the inlet comprises:an outer channel wall (22) extending away from the compressor wheel,surrounding the inlet axis and forming a gas intake portion at alocation axially spaced apart from the compressor wheel, an innerchannel wall (21) within the outer channel wall, the inner channel wallsurrounding the inlet axis, and an annular cavity (23) defined betweenthe inner and outer channel walls, with an annular opening of theannular cavity (23) disposed between the gas intake portion and theleading edges of the blades of the compressor wheel, and wherein saidexhaust gases to be returned to the engine are introduced via the EGRduct into the compressor annular cavity (23).
 8. The system as in claim7, further comprising at least one inducer recirculation slot (25) inthe inner channel wall (21) upstream of the inducer.
 9. The system as inclaim 8, wherein said inducer recirculation slot (25) is substantiallyannular.
 10. The system as in claim 7, wherein the inner channel wall(21) and the outer channel wall (22) are co-axial.
 11. The system as inclaim 7, further comprising at least one flow directional control devicelocated in said annular cavity.
 12. The system as in claim 11, whereinsaid flow directional control device includes at least one strut. 13.The system as in claim 7, wherein gas from the EGR tangentially entersthe annular cavity.
 14. The system as in claim 7, wherein the annularcavity comprises a first end and a second end axially spaced apart fromthe first end, the annular opening is disposed at the first end, and thesecond end is axially disposed in the vicinity of the free edges of theblades of the compressor wheel.
 15. The system as in claim 14, whereinthe recirculation duct opens into the annular cavity at a locationdisposed between the annular opening and the second end.
 16. The systemas in claim 14, wherein the second end is a blind end.